A Three-State Nanofluidic Field Effect SwitchMarie Fuest, Caitlin Boone, Kaushik K. Rangharajan, A. Terrence Conlisk,* and Shaurya Prakash*

Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, Ohio 43210, United States

*S Supporting Information

ABSTRACT: We report a three-state nanofluidic field effectswitch in an asymmetrically gated device with a forward(positive), off (zero), and a reverse (negative) current state fortunable control of ionic transport by systematically controllingthe gate potential. The embedded gate electrode allows formodulation of the ionic current through the 16 nm deepchannels as a function of electrolyte concentration and gateelectrode location for a fixed streamwise potential.

KEYWORDS: nanofluidics, field effect, nanochannel, gating, fluidic transistor, fabrication

Nanofluidic field effect devices, analogous to semi-conductor field effect transistors, feature a nanofluidic

channel with a “gate” electrode embedded in the nanochannelwall for systematic manipulation of the surface potential at thedielectric−electrolyte interface.1,2 The gate electrode is isolatedfrom the aqueous electrolyte in the channel by a dielectric layer.An axial or streamwise potential applied between the inlet andoutlet of the nanochannel drives streamwise transport of ionsthrough the nanofluidic channel.3−5 An independentlycontrolled applied voltage to the gate electrode allows active,tunable control over the local surface potential in thenanofluidic channel. Taking advantage of the nanoscalegeometry, the local change in potential penetrates throughthe entire nanoscale depth of the channel6 permitting activeand reconfigurable control7 over the concentration of chargecarriers, or net space charge density, in the fluid volume.8,9 Thisactive control over the concentration of ionic species modulateselectrokinetic flows as measured by changes in current throughthe nanofluidic architecture (nanopore or nanochan-nel).7,8,10−13 An increase in current indicates a higherconcentration of charge carriers flowing through the nano-channel and a decrease in current indicates a lowerconcentration of charge carriers flowing through the nano-channel.7,8

Nanofluidic field effect transistors, or flow-FETs, have beenproposed as the basis for ionic circuits,8,14 with the goal ofperforming detailed logic functions similar to solid-state circuitsfor controlled transport of ionic species in small volumes(nanoliter or less) of fluid. Many technological demonstrationshave shown manipulation of ions and biomolecules atfemtoliter volumes8 for biosensing,15,16 sample concentration,17

molecular sorting,18 and separations19,20 moving toward the stillelusive goal of performing controlled ion transport functionssimilar to biological systems.7,21−23 Most advances in develop-ment of ionic circuit elements have focused on ionic diodes andcurrent rectifiers.9,24−30 Nearly all nanofluidic diodes operate ina fixed rectification regime with no changes possible torectifying properties after device fabrication. A recent

demonstration of tunable, reconfigurable rectification with ananofluidic field effect device demonstrates the advantage ofusing flow-FET devices for building elements of ionic circuitry.7

In contrast to diodes that permit flow of current and thereforeions in a preferential direction, based on the axial potentialdirection, current switching, or the ability to switch the current“on” and “off” arising from gate control for fixed axial potentialhas not yet been reported. Switching or flow gating is a keyadvance required for ionic circuits in moving toward fluidiclogic control as well as specific applications such as artificial ionchannels.15,31

This letter reports on the implementation of a nanofluidicfield effect device with 16 nm deep channels as a current switchcontrolled by systematically manipulating the potential appliedto the gate electrode. All measured current states werereferenced with respect to the ungated case, that is, the casewhere only an axial potential was applied but no gate potentialwas applied. The device reported in this paper is in anasymmetric configuration (Figure 1), with details discussedlater. A positive current increasing with applied gate voltage(forward state) and a negative measured current (reverse state)for a fixed axial voltage were observed in addition to a devicestate where the measured current was nearly zero (off state)during the gating of the nanofluidic device. Therefore, thepurpose of this letter is to demonstrate the operation of ananofluidic field effect switch controlled by a gate electrodeacross a broad range of experimental parameters.In recent years, several other reports have demonstrated the

complexity and challenges in fabricating gated nanochannelswith insulated gate electrodes.8,10,11,32,33 We developed analternate fabrication sequence5 for gated nanofluidic devicesthat relies on photolithography, wet etching, and oxygen plasmabonding techniques34−36 to yield a sealed nanofluidic device.The nanofluidic device consists of two microfluidic channels as

Received: December 2, 2014Revised: February 22, 2015Published: March 2, 2015

Letter

pubs.acs.org/NanoLett

© 2015 American Chemical Society 2365 DOI: 10.1021/nl5046236Nano Lett. 2015, 15, 2365−2371

fluidic reservoirs for a bank of three 16 nm deep × 30 μm wide× 5 mm long nanofluidic channels etched in borosilicate glass(Figure 1). The low aspect ratio5 of the nanofluidic channels(depth ≪ width) enables nanoscale effects similar to biologicalsystems while providing high throughput and benefits of 1-Dtheoretical analysis.15,37 Gold (Au) gate electrodes (20 nm high× 25 μm wide) were patterned on a second glass substrate thatacts as a cover for the etched channels (Figure 1A). The Augate electrodes were asymmetrically placed along the length ofthe nanofluidic channel to allow investigation of the effect ofelectrode location on the field effect control, or currentmodulation. A polydimethylsiloxane (PDMS) layer spun overthe Au electrodes isolated the gate electrodes from theelectrolyte within the nanochannels and provided a surfacefor bonding the two substrates (one with channel featuresetched in borosilicate glass and the cover with the isolated gateelectrodes). Detailed fabrication procedures for the channelnetworks and device bonding were reported previously.5 Aschematic showing the exploded view of the fabricated device isshown in Figure 1A. A scanning electron microscope (SEM)image of the nanochannel cross section is shown in Figure 1B,showing the PDMS dielectric layer was ∼500 nm thick. Theexploded side view schematic (Figure 1C) shows the glasscover with asymmetrically spaced Au gate electrodes isolatedfrom the fluid by a PDMS dielectric layer. The cut-line alongA−A′ was used to generate the side view schematic and isindicated by the red dash-dot line on the full device schematic

(Figure 1A). The device featured six individually addressablegate electrodes with each electrode location given by therelative distance from the nanochannel inlet to the center of thegate electrode (see Supporting Information Figure S1). In thiswork, only one of the six electrodes was active at a given timeand the results are described in the sections to follow.In line with previous reports,8,38 electrolyte solutions of KCl

in DI (deionized) water at concentrations varied between 0.1mM to 100 mM at pH 7 ± 0.2 were used as the working fluid.A potential difference between the two microchannel reservoirs(axial potential, Va), drives flow through the nanofluidicchannels as shown schematically in Figure 1D. The netdirection of fluid flow for the ungated case (gate voltage, Vg = 0V), indicated by the red arrow in Figure 1D, is in the samedirection as the net transport of K+ ions.39 Since the electricalresistance of the microchannel was significantly lower than thenanochannels (order of MΩ for the microchannels comparedto GΩ for the nanochannels, as measured for channels filledwith 1 mM KCl), each microchannel was considered essentiallyequipotential for subsequent analysis. A second power supplywas used to apply the gate potential to the embedded gateelectrode, where all potentials were applied with respect to thesame ground for a common reference. Current through thenanofluidic channels was monitored using a picoammeter. Eachdata point for measured current is the average of 10 currentmeasurements with error bars representing one standarddeviation in the mean. Leakage current through the PDMS

Figure 1. (A) Schematic showing an exploded view of the fabricated field effect nanofluidic device. The fluidic network consisted of two 10 μm deep× 50 μm wide × 3.2 cm long microchannels connected by a bank of three 16 nm deep × 30 μm wide × 5 mm long nanofluidic channels wet-etchedinto borosilicate glass. Gold gate electrodes (20 nm high × 25 μm wide) were patterned on a second glass substrate (cover) and bonded to thesubstrate with the channel network using an intermediate PDMS layer. The PDMS layer isolated the gate electrodes from the fluid in thenanochannels. (B) Scanning electron microscope (SEM) image of the bonded nanochannel cross section. The complete nanochannel width of 30μm does not permit imaging the entire cross section in one frame. From the SEM image, the PDMS dielectric layer was measured to be ∼500 nmthick. (C) Exploded side view of the device. The red dash-dot line in panel A indicates the cut-line (A−A′) used to generate the side view schematic.(D) Assembled side view schematic showing the experimental setup with the applied axial (Va) and gate (Vg) potentials. Only one electrode is shownas only one electrode was active at a given time. The direction of fluid flow in the ungated case (Vg = 0 V) is marked by the red arrow. The axial andgate potentials are referenced to the same ground. The location of a given electrode is the relative distance of the center of the electrode from thenanochannel inlet as indicated in the schematic by L1.

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dielectric layer was also measured to confirm adequate electricalisolation of the gate electrodes in accordance with previousreports for gated nanochannels (Supporting Information FigureS2 and S3).7,10,32,33

The nanofluidic device was first operated with Vg = 0 V, alsoreferred to as the ungated case. Nanochannel conductance, G,given by the ratio of the measured current to applied axialpotential (I/Va), follows a linear trend for high concentrations(≥ ∼1 mM) as expected from bulk fluid conductance.1,7,8,10,40

At low concentrations (≤ ∼1 mM) conductance has nearly aconstant value (Figure 2), indicating the surface charge

governed regime.8,10,40,41 To maintain electroneutrality, thenet charge in the nanochannel volume must be equal andopposite to the total charge on the nanochannel walls. Thenumber of charge carriers in the nanofluidic channel, andtherefore, the conductance is determined by the surface chargeat low concentration. The critical concentration where thetransition between the bulk transport and surface chargegoverned transport occurred was ∼1 mM in agreement withtrends reported previously.7,8,40

The height of the bonded nanofluidic channels was estimatedfrom the conductance data in the bulk conductance or highconcentration range.7 The estimated height from conductancedata was 18 nm where the bulk conductance for three 18 nmdeep, 30 μm wide, 5 mm long nanochannels is plotted in Figure2 indicated by the solid black line. The equation for the bulkconductance is presented in the Supporting Information. Theestimated depth is in reasonable agreement with atomic forcemicroscopy measurements that showed the nanochannel depthbefore bonding was 16 nm ± 1 nm.5

Applying a gate voltage causes a change or modulation ofmeasured current through the nanofluidic device with respectto the ungated case (Vg = 0 V). As a representative case, currentthrough the nanochannels as a function of the gate voltage for 1mM KCl solution at an axial potential of 3 V is reported inFigure 3A. The gate electrode was positioned at 0.57 L, whereL is the total length of the nanochannel (Figure 1D).As the gate voltage was increased from 0 to 2 V, current

through the nanochannel increased from 0.106 nA to 0.220 nA.

As the gate voltage was swept from 0 V to −2 V the measuredcurrent decreased until the current was switched off, that is, nomeasurable current was recorded at Vg = −1.4 V with Va still at3 V. Changing the gate voltage from −1.4 V to −2 V reversedthe polarity of the measured current with a current of −0.026nA for Va = 3 V and Vg = −2 V. Therefore, the device has threestates of operation with respect to the ungated case; a forwardcurrent, an off or zero current case, and a reverse current statefor a fixed axial potential.To our knowledge, this device presents the first demon-

stration of current switching in a nanofluidic field effectconfiguration, with tunable control over both the magnitudeand direction of current. Figure 3B shows repeatable on−offoperation of the nanofluidic field effect switch. For fixed axialpotential, the gate voltage can be tuned to modulate the currentto the desired value. For example, in Figure 3B the appliedvoltages for 0.1 mM KCl were cycled from Va= 3 V, Vg = 0 Vproducing 0.024 ± 0.002 nA of current to Va = 3 V, Vg = −3 Vto switch off the current. Although the details on concentrationdependence of measured current modulation with Vg arediscussed later, it is worth noting here that in the surfacecharged governed regime (typically with electrolyte concen-

Figure 2. Nanochannel conductance for the ungated case is shown as afunction of KCl concentration. Conductance was independent ofconcentration at “low” concentration and follows a linear trend at“high” concentrations in agreement with previous reports.8,40,41 Thetransition between these two regimes occurs near the criticalconcentration of ∼1 mM, with general agreement in published reportsthat the low concentration regime (<1 mM) is the surface chargegoverned transport regime for nanofluidic channels. The equationused for the bulk conductance line is given in the SupportingInformation.

Figure 3. (A) Plot showing the measured current as a function of gatevoltage with the overall trend showing evidence for operation of ananofluidic field effect device as a three-state ionic current switch. Acurrent of 0.106 nA was measured through the nanofluidic channelsfor Va = 3 V, Vg = 0 V. For Vg = 0 V to −2 V, the measured currentdecreased from 0.106 nA to −0.026 nA with no measurable current atVg = −1.4 V. Here, +2 V applied to the gate electrode results in 0.220nA of current. From the ungated conductance for 1 mM KCl (Figure2), achieving the same current, that is, 0.220 nA purely from Varequires Va = 7.0 V. (B) Current switching for 0.1 mM KCl indicatingthe gate voltage can be tuned to produce the desired current. Thecurrent is 0.024 ± 0.002 nA for Va = 3 V, Vg = 0 V, and switched off forVa = 3 V, Vg = −3 V.

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tration ≤ 1 mM for the ungated case), controlling the gatepotential allows switching of the current.Using previously reported analytical models to explain the

observed data trends for electrokinetically driven flow,7,10,42 thecomponents of the current due to electromigration of K+ andCl− are given by

μ⃗ = ⃗++ +I F A E( [K ] )K K (1)

and

μ⃗ = − ⃗−− −I F A E( [Cl ] )Cl Cl (2)

where, Ii⃗ is the current due to a species i, μi is the ionic mobilityof species i, A is the cross section area of the nanochannel, [i] isthe concentration of species i in the nanochannel, and E⃗ is theelectric field.7,42 The ionic mobility, μi is given by ziDiF/RTwhere zi is the valence of species i, Di is the diffusion coefficientof species i, F is Faraday’s constant, R is the universal gasconstant, and T is the temperature.39 Note that the term inparentheses in the equations above is the flux of the ions underthe influence of an electric field.42 Applying the previouslyreported assumption, μK+ ≈ −μCl−,7 the total current due to thesummation of the cationic and anionic contributions is given by

μ⃗ = + ⃗+ −+I F A E([K ] [Cl ])total K (3)

The 5 mm long nanochannels (compared to 30 μm widthand 16 nm depth) are sufficiently long such that the electricfield in the ungated case can be approximated as E = Va/L, ashas been reported previously for nearly 1-D nanochannels.43

Consequently, any change in current for a finite gatepotential compared to the ungated case will be caused by achange in the ionic concentration or a change in the electricfield within the nanofluidic channel, or both (eq 3). Since thegate electrode is known to induce a local change in the effectivesurface charge on the channel wall,9,44,45 to maintain overallelectroneutrality of the system, the relative concentration ofpotassium and chloride within the nanofluidic channel mustchange.7−9 Karnik et al.8 experimentally verified this relativechange in concentration of ionic species through fluorescencemeasurements. The consequence of this change in relativespecies concentration is the modulation of the current due toincreased or decreased availability of charge carriers.8,9 Inaddition to the change in the total number of charge carriers inthe channel, it is likely with the gate voltage turned on, the localelectric field modulation also causes a change in the localconcentration, which will influence the flow of current. Furtherinvestigation of the nature of concentration profiles is needed,46

though past experimental work has also implied influence ofconcentration gradients on current profiles,7,44 which caninclude the possibility of negative current.The change in concentration is often analyzed in terms of the

charge stored by the gate electrode/dielectric/electrolytecapacitor,1,7,47 approximating the flow-FET as an equivalentelectric circuit for analysis of data trends.1,7 In these previousreports, the electric field through the nanochannel is consideredto be the same in the ungated and gated cases. Although changein the species concentration will result in current modulationand is required to maintain overall electroneutrality, given thesign of each term in eq 3 reversal of the current from positive tonegative, as observed in Figure 3, requires a change in thedirection of the electric field. The determination of the exactmechanism for this expected reversal of electric field within thenanochannel will likely require detailed numerical models as

discussed below and is beyond the scope of this work. Ahypothesis based on discussion of relevant literature and furtherdevice characterization is presented next.Detailed numerical models have shown that the potential of

the gate electrode can penetrate through the entire depth of thenanochannel.6,48 The gate electrode descreening modelpresented by Liu et al.6 suggests that ionic flux driven by theaxial potential through the nanochannel along with high electricfields (on the same order as the thermal voltage) prevent theformation of an electric double layer (EDL) in the electrolyteregion immediately below the gate electrode, leaving the gateelectrode partially “descreened.” The result is a local region inthe nanochannel with a potential proportional to the gateelectrode that penetrates throughout the depth of thenanochannel. The partial descreening of the gate electrode isone proposed explanation for why experimentally observedcurrent modulation exceeds predictions of capacitive or electricdouble layer based models.6,49 Additional numerical reports ontwo-dimensional analysis of gated nanofluidic channels havealso shown that the electric field in the nanochannel issignificantly more complicated by applying a potential to thegate electrode9,44,45 than is captured by analytical models. Theelectric field throughout the nanofluidic architecture is afunction of both applied axial and gate potentials as well as gateelectrode location, which alters the impact of the field effect onthe measured current.45,50 However, many challenges remain innumerically quantifying the effect of transverse electrostaticsfrom the gate electrode on streamwise transport of ions andfluid for real nanofluidic field effect devices.6,48

In the present case, we hypothesize that the electrolyteregion directly below the gate electrode is at some potentialαVg. The variable α is an empirical parameter that accounts forpotential change from the gated dielectric−electrolyte interfacethrough the depth of the nanofluidic channel to the otherphysical wall of the device (see Supporting Information FigureS4 for a visual representation). It is likely that α depends on avariety of factors including dielectric material,8 dielectricthickness,7,45 gate electrode length,9,44,45 gate electrodeposition,9,44 and electrolyte concentration.6 As α → 0, thesystem approaches the previously reported capacitor modelwhere the gate electrode modulates the ionic concentration inthe channel but the electric fields in the gated and ungatedcases are the same. Therefore, the change in potential withinthe nanochannel in the region below the gate electrode altersthe electric field in the nanochannel and enables the switchingbehavior observed in Figure 3. As real dielectrics in nanofluidicfield effect devices permit a finite gate leakage cur-rent,7,10,32,33,46,47,51,52 in addition to the capacitive coupling ofthe gate electrode and the nanofluidic channel, a finite gateleakage current was measured through the PDMS dielectriclayer. Although the magnitude of the gate leakage current issmaller than the measured ion transport current (seeSupporting Information) the presence of the finite gate leakagecurrent may contribute to the alteration of the electric field,46

likely enhancing the effect of the gate electrode compared tothe case of an ideal dielectric.In order to further test the hypothesis that a change in

electric field induced by the gate electrode is responsible for theobserved three-state current switch, Va, Vg, and L1 (center ofgate electrode as measured from nanochannel inlet) were variedsystematically and the subsequent effect on the measuredcurrent through the nanofluidic channels is reported. Figure 4shows measured current at three values of axial potential (Va).

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For the representative case, KCl concentration was 1 mM andthe gate electrode was located at 0.57 L. Interestingly, for allthree values of Va, the slope in Figure 4, dI/dVg, remainedconstant with a value of 0.060 ± 0.003 nA/V.The slope represents the change in current caused by the

applied gate potential. For the axial potential range reportedhere (1−5 V) the measured current (in the axial or x-direction)can be approximated as the sum of the contributions due to Vaand Vg respectively,

μ

μα

= +

= + +−

+ −

+ −⎛⎝⎜

⎞⎠⎟

I AF E

AFVL

V

L L

([K ] [Cl ])

([K ] [Cl ])

gated gated

gated gateda g

1 (4)

where, [Kgated+ ] and [Clgated

− ] are the concentration of potassiumand chloride ions in the channel in the gated case. Uponapplication of a gate voltage, the concentration of species in thenanochannel will change to maintain overall electroneutral-ity.9,44 It is expected that the change in concentration requiredto satisfy electroneutrality will depend on the magnitude andpolarity of the gate voltage.9,10 The term (Va/L) is the electricfield in the nanochannel in the ungated case43 and the term(αVg/(L − L1)) is the contribution to the electric field from thegate electrode where L − L1 is the distance from the gateelectrode to the grounded microchannel. Note that for Vg = 0 Veq 4 becomes the ungated case with [i] = [iungated]. As α → 0and Vg ≠ 0, eq 4 approaches the capacitor model used inprevious reports1,7 with the same electric field in the gated andungated cases.From eq 4, the contribution to the electric field from the gate

electrode has either an additive or subtractive effect whencompared to the ungated axial field, as all potentials are appliedwith respect to the same reference. Application of a gate voltagewill either enhance the effect of the axial voltage or reduce theeffect, with the possibility of reversal in the electric field basedon the relative magnitude and polarity of (Va/L) with respect to(αVg/(L − L1)). For example, in the case of a positive axialpotential (i.e., Va = +3 V) the contribution to the electric fieldfrom the gate electrode will be in the same direction as the axialfield for a positive gate voltage (+Vg) leading to an increase inelectric field and, therefore, forward current (Figure 5). In the

case of a positive axial potential (i.e., Va = +3 V) and a negativegate voltage (−Vg), the contribution due to the gate electrodewill be opposite to the axial field leading to a decrease in electricfield and, therefore, current.When the sign of the axial potential is reversed (i.e., Va = −3

V) the relative impact of the gate potential will also be reversed,that is, a (+Vg) decreases the magnitude of the field while anegative gate potential (−Vg) increases the magnitude of thefield. This leads to a decrease in current (i.e., smaller value ofnegative current, Figure 5) for Va = −3 V and Vg > 0 and anincrease in current for Va = −3 V and Vg < 0 (i.e., larger value ofnegative current). In Figure 5, zero current was measured at Va= +3 V, Vg = −1.4 V, and Va = −3 V and Vg = +1.8 V,consistent with modeling results of Ai et al.50 that showedchanges to the electric field induced by the gate electrodedramatically decreased ionic current when Va and Vg were ofopposite sign. In further support of the proposed hypothesis,current modulation in the nanofluidic field effect switch followsthe expected dependence on relative axial and gate potential.Considering the effect of changing the gate location, as L1 is

increased from the center of the channel (0.50 L) toward thegrounded microchannel (L), the contribution of the gateelectrode to the electric field, (αVg/(L − L1)), is expected toincrease. This means that for a given axial potential (Va) thechange in current induced by the gate electrode will increase asL1 increases. Figure 6 shows measured current at fixed axialvoltage for a gate voltage sweep from −2 to +2 V for threerepresentative electrode locations of 0.43 L, 0.48 L, and 0.57 L.The current modulation, quantified by the slope dI/dVg, was0.003 nA/V, 0.014 nA/V, and 0.060 nA/V for 0.43 L, 0.48 L,and 0.57 L respectively. The increased current modulation withL1 indicates increased contribution to the electric field by thegate electrode, as expected.Analysis based on electromigration similar to previous

reports7,10,42 allows qualitative prediction of observed datatrends for various ranges of Va, Vg, and L1. However, such ananalysis based on electromigration alone does not considereffects of local accumulation and depletion of ions andsubsequent changes in species concentrations. As a concen-

Figure 4. Representative plot showing current modulation in ananochannel filled with 1 mM KCl as a function of gate voltage forthree test axial potentials. In all three cases the slope, dI/dVg, remainedconstant with a value of 0.060 ± 0.003 nA/V. It is worth noting that inthe same device for a given electrolyte concentration, the measuredcurrent can be switched off for different combinations of Va and Vg. Inaddition, current reversal was also be obtained by systematically tuningVg with respect to Va.

Figure 5. Representative plot demonstrating that the currentmodulation depends on the additive or subtractive effect of theelectric field induced by the gate electrode with respect to the axialfield. The gate electrode was located at 0.57 L. For Va = +3 V anegative gate voltage (−Vg) decreases the magnitude of the currentwhile for Va = −3 V the same negative gate voltage (−Vg) in creasesthe magnitude of the current. Here, current was switched off for Va =+3 V with Vg = −1.4 V and Va = −3 V with Vg = +1.8 V, consistentwith modeling results that showed changes to the electric field inducedby the gate electrode dramatically decrease ionic current when Va andVg were of opposite sign.50

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tration gradient in the nanochannel is needed to maintainsteady-state current and flux balance for incompressible flow,detailed modeling or experimental determination of actual iondistributions and analysis of the effect of these gradients on thecurrent in gated nanofluidic channels will be useful and is thesubject of future work.The observed current modulation is expected to depend on

bulk KCl concentration.6,10 Figure 7 shows a summary of gating

the nanochannels for 0.1 mM to 100 mM KCl at pH 7 ± 0.2.Due to differences in conductance for various concentrations,the current in Figure 7 is reported as a dimensionless current,ΔI/I0 = (Igated − Iungated)/Iungated. The dimensionless numberΔI/I0 is the change in current due to the gate compared to thecurrent in the ungated case. The current off state (Igated = 0 nA)corresponds to a dimensionless number of −1. A dimensionlesscurrent of 0 indicates the ungated case (Vg = 0 V, Igated =Iungated).Since switching the current off requires ΔI/I0 = −1, as bulk

concentration increases higher gate voltage is required toachieve the off current state. The change in current induced bythe gate electrode (ΔI) is smaller relative to the ungatedcurrent (smaller ΔI/I0) as bulk KCl concentration increases.Decrease in current modulation with increasing bulk KClconcentration is attributed to enhanced screening of the gateelectrode by the KCl solution at higher concentrations. At lowconcentration, a gate voltage of −1.4 V and −3 V was requiredto switch off the current for 1 mM KCl and 0.1 mM KCl

respectively (demonstrated in Figure 3). The lower currentmodulation for 0.1 mM KCl compared to 1 mM KCl is likelydue to increased availability of charge carriers in the 1 mM caseas with highly overlapped electric double layers for the 0.1 mMcase, the co-ions are excluded from the nanochannels.7,53

A three-state nanofluidic field effect current switch, with aforward current, an off or zero current, and a reverse currentstate has been demonstrated. For a fixed streamwise potential,the electric field within the nanofluidic channel is altered bysystematically controlling the potential applied to a gateelectrode embedded in the nanochannel wall. The change inelectric field within the nanochannel allows for tunable controlof the current through the nanochannels as a function ofelectrolyte concentration and location of gate electrode.Development of a current switch represents a key addition toexisting ionic circuit elements toward the development offluidic logic circuits.

■ ASSOCIATED CONTENT

*S Supporting InformationMeasurement of the gate electrode location, gate leakagecurrent, a visual representation of the proposed hypothesis,further details on the experimental methods, and bulkconductance equation. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: conlisk.1@osu.edu.*E-mail: prakash.31@osu.edu.

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge the staff at Nanotech WestLaboratories for assistance with equipment during fabricationand characterization of devices. We also thank E. Rosenthal-Kim for useful suggestions during the preparation of thismanuscript. The authors would like to acknowledge partialfinancial support from the U.S. Army Research Office (ARO)through grant W911NF09C0079, and the National ScienceFoundation (NSF) through grant CBET-1335946. M.F. would

Figure 6. Current modulation increases as the relative distance of the gate electrode from the channel inlet, L1, increases, in agreement with theproposed hypothesis. The increase in modulation for the representative case of 1 mM KCl (Va = 3 V) is demonstrated by the increase in the slope asshown in the plots above going from the left to right panels.

Figure 7. Plot shows dependence of current modulation on bulk KClconcentration for a representative axial potential of Va = 3 V. Errorbars were smaller than the markers and are therefore not shownexplicitly. Current modulation was observed to decrease as bulk KClconcentration increases likely due to enhanced screening of the gateelectrode.

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like to thank NSF graduate research fellowship program(GRFP) for support.

■ ABBREVIATIONS

SEM, scanning electron microscope; PDMS, polydimethylsilox-ane; DI, deionized; EDL, electric double layer; GLC, gateleakage current

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Nano Letters Letter

DOI: 10.1021/nl5046236Nano Lett. 2015, 15, 2365−2371

2371